1. Field of the Invention
This invention relates to optoacoustic modulators that are used for optical communications, optical measurements and lasers. This application is based on Patent Application No. Hei 9-235219 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
The optoacoustic modulators are one kind of optical modulators, which work as follows:
Modulation signals are applied to piezoelectric elements, which in turn generates ultrasonic waves. Using the ultrasonic waves, refractive indexes of optical media are periodically vibrated, so that modulation is effected on light beams.
FIG. 8 is a block diagram showing an example of a configuration of the optoacoustic modulator. The optoacoustic modulator of FIG. 8 is configured by an input-side optical fiber 51, a lens 52, an oscillator 53, a piezoelectric vibrator 54, a prism 55, a lens 56, an output-side optical fiber 57 and an optoacoustic element 58.
In FIG. 8, the oscillator 53 applies high-frequency signals to the piezoelectric vibrator 54 attached to the optoacoustic element 58, so that periodical variations occur in refractive index of the optoacoustic element 58 due to ultrasonic waves. Thus, diffraction grating is formed.
Incoming beams incident on the input-side optical fiber 51 are converted to parallel beams by the lens 52. These beams are incident on the optoacoustic element 58.
Then, the above beam is subjected to diffraction by the diffraction grating, which is formed inside of the optoacoustic element 8. Thus, it is divided into transmitted beam and primary (or first-order) diffracted beam. The primary diffracted beam is transmitted through the prism 55 and is converged by the lens 56, from which it is output by means of the output-side optical fiber 57.
By turning the high-frequency signals applied to the oscillator 53 on and off, it is possible to activate or cancel formation of the diffraction gating in the optoacoustic element 58. Thus, it is possible to turn on and off the beams which are transmitted between the input-side optical fiber 51 and the output-side optical fiber 57.
Namely, the aforementioned configuration of FIG. 8 actualizes function of the optical switch or optical modulator. Because of such a function, the optoacoustic modulator is widely used in the fields of the measurement devices and the like.
In the case where the aforementioned optoacoustic modulator is used for the measurement device, however, there occurs a problem due to loss-wavelength dependency characteristic of the optoacoustic modulator, wherein loss of the optoacoustic modulator highly depends on wavelength of input light.
FIG. 9 shows an example of the loss-wavelength dependency characteristic of the optoacoustic modulator. By changing the wavelength of the light transmitted through the optoacoustic modulator, it is possible to observe ripple characteristic in loss-wavelength dependency characteristic, for example, which is shown in FIG. 9. Such ripple characteristic is caused by "PMD" (an abbreviation for "Polarization Mode Dispersion") due to birefringence of the optoacoustic element 58.
When the light, which have the PMD while being transmitted through the optoacoustic element 58, is input to the output-side optical fiber 57, interference is caused to occur due to mixing effect in the optical fiber 57. Herein, a degree of interference depends on the wavelength of the light.
So, by changing the wavelength of the light to be transmitted through the optoacoustic modulator, there occurs power variations at an end of the output-side optical fiber, regardless of the constant power of the input light. That is, the ripple characteristic emerges on variations of loss of the optoacoustic modulator.
As described above, if the ripple characteristic exists in the loss-wavelength dependency characteristic of the optoacoustic modulator, the aforementioned problem occurs in the optoacoustic modulator, which is used for the measurement device and the like.
In the aforementioned optoacoustic modulator, only one optoacoustic media is used to diffract light signals input thereto. For this reason, as compared with an amount of crosstalk at a static state, an amount of crosstalk at a high-speed modulation mode is deteriorated remarkably. Such deterioration of the amount of crosstalk causes a problem, which cannot be bypassed by the known usage manners of the optoacoustic modulators.